1. Field of the Invention
The present invention relates to a method of preparing a coating solution, the coating solution prepared by this method, and corresponding use of the coating solution for coating a substrate and the coated substrate.
2. Description of the Related Art
Spin-coating is a well known technique to coat a substrate. The coating of a substrate by spin-coating methods depends on several parameters. The problem underlying the present invention will now be described referring to the coating of a wafer, especially a 300 mm wafer. However, it is not intended to limit the invention to the coating of a wafer.
Spin-coating of a wafer highly depends on the size of a wafer and the viscosity of the coating solution. For wafers of a size of 100 mm there are well known techniques to coat different coating materials, e.g. a photoresist or a sol-gel solution, onto the wafer. While scaling up the wafer to a 300 mm wafer, many problems arise from the enlarged diameter of the wafer. The differences in the angular speed from the center to the boarder of the wafer are much higher compared to 100 mm wafers. The coating solution to be applied to the wafer thus has to be highly wettable to allow for a constant coating of the substrate. Further, the solution must not dry at the outer rim of the wafer too quickly in order to avoid inhomogeneities in the constitution and thickness of the coating throughout the surface area of the wafer. This is a problem especially in sol-gel solutions containing a high solid content.
Further, the stress within the coating layer becomes a more serious problem as the size of the wafer increases. Current layers of sol-gel coatings introduce a high stress of 300 MPa and higher. By this high stress, the large wafer is bent more than a small one. Moreover, the larger the surface area of the wafer, the greater inhomogeneities regarding the composition and thickness of the layer will develop while coating the wafer.